Conventionally, a rubber plug as disclosed in JP H11-329567A (Patent Document 1) is known as a rubber plug for waterproofing electrical wires collectively. This rubber plug has multiple electrical wire insertion holes for the passage of respective electrical wires. The electrical wires and the rubber plug are sealed by intimate contact between the outer circumferential surfaces of the electrical wires and the inner circumferential surfaces of the electrical wire insertion holes. Also, the rubber plug and its housing are sealed by intimate contact between the outer circumferential surface of the rubber plug and an inner wall surface of a rubber plug mounting opening of the housing.